26/08/2025
1) Quick formulas (keep these handy)
Demand kW (per category) = Connected kW Γ Demand factor
Sum Demand (kW) = Ξ£ Demand kW
After Margin (kW) = Sum Demand Γ (1 + Future margin %)
Required kVA = After Margin Γ· Power factor (pf)
3-phase line current (A) at voltage V (e.g., 415 V):
I = \frac{\text{kVA} \times 1000}{\sqrt{3}\times V}
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2) Typical demand factors (common practical values)
Lighting: 0.7 β 0.85 (use 0.8 typical)
Fans: 0.6 β 0.8 (use 0.7 typical)
AC (split/chiller): 0.6 β 0.8 (use 0.7 typical)
Sockets/outlets: 0.4 β 0.6 (use 0.5 typical)
Lifts / Motors / Fire pumps: 1.0 (consider full load)
Misc / server loads: 0.8 β 0.9 (server pf/usage higher)
Defaults I use in examples: PF = 0.90, Future margin = 25% (adjust to your policy).
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3) Worked example (step-by-step)
Assume these connected loads (kW):
Lighting = 50
AC = 150
Fans = 20
Lifts = 20
Pumps = 15
Sockets = 30
Misc = 10
Apply demand factors (chosen typical values):
Lighting: 50 Γ 0.8 = 40.0 kW
AC: 150 Γ 0.7 = 105.0 kW
Fans: 20 Γ 0.7 = 14.0 kW
Lifts: 20 Γ 1.0 = 20.0 kW
Pumps: 15 Γ 1.0 = 15.0 kW
Sockets: 30 Γ 0.5 = 15.0 kW
Misc: 10 Γ 0.8 = 8.0 kW
Sum Demand = 40 + 105 + 14 + 20 + 15 + 15 + 8 = 217.0 kW
Add future margin 25%:
After Margin = 217 Γ 1.25 = 271.25 kW
Convert to kVA (pf = 0.90):
Required kVA = 271.25 Γ· 0.90 = 301.39 kVA
Pick the next standard transformer size β 315 kVA.
Compute LV side line current at 415 V:
I = 301.39 Γ1000 Γ· (β3 Γ 415) β 301,390 Γ· 718.6 β 420 A
(So choose LV switchgear/cables rated accordingly, consider inrush, protection, and derating.)
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4) Practical notes & checks
Always check the largest motor start current β large motors and lifts may require transformer inrush / subtransient considerations.
If heavy harmonic loads (UPS, big drives, server rooms) exist, derate or use K-factor transformers or add harmonic mitigation.
For PV export or EV chargers: treat them as additional loads or negative export depending on protection and utility rules.
Confirm local standard transformer ratings with your utility/stock (100, 160, 200, 250, 315, 400, 500, 630β¦ kVA).
For medium voltage primary (e.g., 11 kV) sizing, convert LV kVA to MV side using appropriate tap and short-circuit considerations and coordinate protection with the utility.
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